Playing with
groups.google.com
I understand that <<Unix window managers do not have such a "Always on top" feature. To be more precise, most Unix window managers can do something like
"Always on top", but there's no X11 standard.>> (from Slaven Rezic)
From Slaven, the (untested) trick is:

I am assuming you are looking to make a global grab ... somthing that will not let the user switch application's, correct ? Well, i never suggest this, as if your program hangs you may well roger the user ... After the warning, take a look at the $widget->grabGlobal() method. I do suggest the grab() method, as it is local. Adding this with the general focus methods/properties should do what you want.
from the frivolous to the serious

No, I did not intended to have input only for my app, I was searching for a way to display a small button that will not be obscured by other applications, so a user will be able to see my button (and press it at any time) while being in another application. Something like floating button-bar.

You can call the method geometry on all other toplevels. Than you can type to determine the coordinates from these by taking the lowest X value and the lowest Y value, and add the X size=(highest X - lowest X) and
the same for the Y size.

Than you can call the method geometry on the newly created
Toplevel with these new parameters.

In most window managers, the newly created Toplevel will than be above the older onces.